This invention relates to polymer films suitable for slitting into yarns and weaving into fabrics. The production of slit film tape yarns is well known and complete production lines are offered by numerous machinery manufacturers. Slit film tape yarns are commonly used in the production of woven carpet backing, woven geotextiles, woven bags or sacking and concrete reinforcement. A typical raw material for these products is between about a 3 to 4 melt flow index homopolymer polypropylene. For certain products, a 1 melt flow index polymer is used.
One exemplary conventional production method consists of extruding molten polypropylene through a flat die in the form of a molten sheet. The molten sheet thus formed is then rapidly cooled (quenched) in a temperature controlled cold water bath (quench tank) or via chilled casting rollers to form a solid sheet. Dimensions of this sheet are typically in the range of 4 to 12 mils (0.004-0.012 inches) in thickness and range from about 40 to 80 inches in width. Typically, the thickness is determined by the desired end product, and the width is determined by the width of the die.
Following the quenching process, the now solid sheet passes by a vacuum slot system to remove residual water droplets. Subsequently the sheet passes through a blade bar, which typically comprises a plurality of sharp blades, similar to razor blades. These blades are typically spaced from between about 70 to 150 mils (0.075-0.150 inches) apart. Depending on the desired product and width of the formed sheet, there will be from about 400 to 900 individual yarns produced during passage through the blade bar. Typical linear production speed at this point is between about 100 to 200 feet per minute. At this point in the exemplary production process, the film or the slit yarns have not been oriented (drawn) appreciably.
Following passage through the blade bar slitting system, the sheet of undrawn slit film yarns is passed over a slow speed group of rollers, and then passed through an oven that is heated to a desired temperature using hot air circulated by at least one fan, such as a high speed fan. At the exit of the oven, the heated sheet is passed onto a second group of rollers that are run at a substantially higher speed than the slow speed group of rollers. The speed differential between the respective groups of rollers will typically be anywhere from between a 4 to 1 ratio up to about a 10 to 1 ratio, depending on the process conditions, polymer and desired end product. The speed differential is commonly referred to as the “draw ratio.” Typically, as the draw ratio increases, the width and thickness of the slit film tapes is reduced.
In order to produce a finished yarn with desirable physical properties, the now drawn yarns are passed over annealing rolls, which are typically a series of heated rollers with independent motor drives. By using a combination of temperature and speed, the yarn is allowed to relax, i.e., shrink, from between about 3% to about 20%. The shrinkage is controlled by reduction of roller speeds as the yarns pass through the annealing roller system. Typical production lines contain from between about 3 to 9 heated rollers in the annealing section. Prior to exiting, the fully drawn and annealed yarns pass over chilled cooling rollers to set the properties of the yarns. During the process of annealing, tensile is reduced, elongation is increased and shrinkage due to exposure to hot air is significantly reduced. In order to produce slit film yarns with the desired properties, a particular combination of quenching temperature, draw ratio, draw temperature, annealing temperature, and percent relaxation is required.
The drawn and annealed yarns leave the cooling rolls and are wound up on a multitude of traverse type spool or bobbin winders. In this step, a core or spool is placed onto a spindle which begins to rapidly rotate and the yarn is laced through a reciprocating yarn guide that rapidly moves back and forth across the face of the spool. The yarn is thus laid onto the bobbin. Following a predetermined time or length schedule, the now full bobbins are manually removed, an empty bobbin is placed onto the winder spindle and a new package of yarn is started up. Depending on the yarn dimensions, all of the bobbins will be replaced every 4 to 6 hours. A single yarn or multiple yarns can be placed onto a singly bobbin. In the one exemplary method, a sheet producing 900 individual yarns wound 2 per bobbin will result in a single production line containing at least 450 traverse winders.
The aforementioned production system is typically run as one continuous operation, with polymer resin or pellets being automatically fed into the extruder, the sheet and slit yarns running down the line through the various operations as described above and finishing as a multitude of bobbins of yarn. This process is generally referred to as yarn extrusion. As is known in the art, polypropylene is highly susceptible to stress-induced crystallization, with the higher draw ratios generating a highly crystalline structure. While the yarns are quite flexible and robust in the length direction, there is very little strength elongation in the width of the tape, which leads to a brittle material that will easily fibrillate, or split lengthwise, under stress.
An alternative to the production method described above is to produce oriented film, roll the oriented film onto mandrels, followed by slitting off line. Industrial processes using this approach that are well known include recording tapes, adhesive tapes, strapping and electronic capacitor insulators. United States patents relating to polypropylene based materials include U.S. Pat. Nos. 5,724,222 to Hirano, et al. and 6,094,337 to Ueda, et al., which describes file production for capacitors; U.S. Pat. Nos. 3,394,045 to Gould and 4,495,124 to Van Erden, et al., which describe strapping manufacture and U.S. Pat. No. 6,326,080 Okayama, et al., which describes film production for packaging materials. An exemplary description is as follows.
In this example, after the molten sheet from the extruder is cooled, the production process comprises orienting and winding the film onto a jumbo film winder. These winders can typically produce rolls 120 inches wide and 60 inches in diameter that weigh upwards of 10,000 lbs. In some of these conventional manufacturing processes, the jumbo rolls of oriented film are subjected to one or more subsequent coating operations. For example, adhesive tapes are coated with a sticky material while a recording tape is coated with a metal oxide layer and a protective layer. As a final process, the full width oriented film is passed through a blade bar or other cutting device and converted into a narrower width tapes that exemplarily range from about ⅛ inch wide (audio cassette tapes) up to an inch or more (adhesive tapes).
All of the above mentioned products are biaxially oriented, which means that the film is oriented lengthwise or in the machine direction, as well as being oriented in the transverse or width direction. These two orientation steps can be performed in various sequences as required by the product or production machinery. The width-wise orientation step produces films with excellent cross machine properties. A drawback of width orientation is that these machines are normally exceedingly large, capital intensive and costly to operate.
Numerous patent references describe slitting of oriented plastic films into tapes with properties suitable for use in textile processes. U.S. Pat. No. 4,129,632 to Olson, et al., for instance, describes production of oriented films, slitting into narrow width rapes and winding the resulting tapes onto yarn traverse winders suitable for use in carpet backing applications. In another example, U.S. Pat. No. 3,336,645 to Mirsky describes slitting a sheet of film into tapes and directly winding them onto a beam for use on weaving or warp knitting machines. Further, U.S. Pat. No. 4,137,614 to Wolstencroft uses a modified film feeding layout and threading pattern where multiple rolls of film are slit and the tapes are then wound onto a beam. U.S. Pat. No. 4,906,520 to Kumar describes slitting oriented films to produce tape, and by an unspecified method, the tapes are introduced to a loom to be woven. Finally, U.S. Pat. No. 3,645,299 to Eichler, et al. describes placing a roll of oriented film on the loom with the blade bar mounted on the loom itself.
These exemplary processes noted above offer the advantage of reduced floor space (no traverse winders or beaming creels) and lower labor requirement. However, the processes described all suffer from one or more deficiencies in either materials or process limitation. For example, the use of monoaxially oriented films in these processes is limited to slow slitting speeds, the relatively high cost of biaxially films that are normally required for high slitting speeds, and the like. These deficiencies are serious enough that none of these exemplary processes are currently being operated commercially in the U.S.
It is known that monoaxially oriented homopolymer polypropylene tends to produce brittle products that tend to split lengthwise. Further, it is difficult to produce an acceptable film from a substantially 100% homopolymer polypropylene; nor, would such a film pass through slitter blades at any commercially viable speed. Various methods to reduce the brittle nature of oriented films are known in the art, such as, for example, the blending of random copolymers of ethylene/propylene or ethylene polymers of various densities with the polypropylene. A major drawback to these additive materials is their low melting point, which leads to films that have unacceptably high thermal shrinkage properties for the products described herein. Other examples of materials used to reduce brittleness are described in U.S. Pat. Nos. 5,236,963 to Jacoby, et al. and 6,881,793 to Sheldon, et al. Both of these disclosures address the use of blends of flexible elastomeric type materials to soften and increase extensibility of polymer films.
Further, U.S. Pat. Nos. 4,188,350 to Vicik, et al. and 6,083,611 to Eichbauer, et al. describe production of a multilayer shrink film using an impact copolymer in one or more layers. In another example; U.S. Pat. No. 5,654,372 to Sadatoshi, et al. describes a melt-kneaded process to product an impact copolymer for films. Additionally, U.S. Pat. No. 5,314,746 to Johnson, et al. describes film made from a unique crystalline propylene/ethylene copolymer as the continuous phase with a rubbery propylene/ethylene copolymer s the dispersed phase, in which the continuous phase melts below 160° C.
While attempts have been made heretofore to manufacture polymer films having sufficient cross machine toughness to allow high speed slitting of the films into tape yarns, the art has not provided a facile means or device by which to do so.